Method for the in-process dimensional checking of orbitally rotating crankpins

ABSTRACT

An apparatus for checking the diameter of crankpins of a crankshaft in the course of the machining in a grinding machine includes a Vee-shaped reference device, a measuring device associated with the reference device and including a feeler axially movable along the bisecting line of the Vee-shaped reference device, and a support device with a first arm rotating with respect to a support arranged on the grinding-wheel slide and a second arm rotating with respect to the first and carrying the Vee-shaped reference device. 
     A guiding mechanism includes a limiting device, with mutually abutting surfaces and coupled to the support device, that guides the displacement of the reference device along a trajectory substantially parallel to the profile of the grinding wheel, allows the engagement of the reference device on a crankpin to be checked while the latter is orbitally moving, and does not interfere with the displacements of the support device caused by the orbital motion of the crankpin in the checking condition.

This is a continuation of U.S. patent application Ser. No. 11/014,955,filed on Dec. 20, 2004 now U.S. Pat. No. 7,024,785, which is acontinuation of U.S. patent application Ser. No. 10/657,086, filed onSep. 9, 2003, now U.S. Pat. No. 6,848,190, which is a continuation ofU.S. patent application Ser. No. 10/187,834, filed on Jul. 3, 2002, nowU.S. Pat. No. 6,643,943, which is a continuation of U.S. patentapplication Ser. No. 09/678,379, filed on Oct. 3, 2000, now U.S. Pat.No. 6,430,832, the entire disclosures of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for checking the diameterof a cylindrical part, rotating about a geometrical axis, e.g. anorbitally rotating crankpin, in the course of the machining in anumerical control grinding machine including a grinding-wheel slide,with a reference device for cooperating with the part to be checked, ameasuring device movable with the reference device, a support device forsupporting the reference device and the measuring device, the supportdevice having a support element adapted to be fixed to thegrinding-wheel slide, a first coupling element coupled to the supportelement so as to rotate about a first axis of rotation, and a secondcoupling element carrying the reference device and coupled to the firstcoupling element so as to rotate with respect to it about a second axisof rotation parallel to the first axis of rotation, a control device forcontrolling automatic displacements of the apparatus from a restposition to a checking condition, in the course of such displacementsthe reference device performing closing movements with respect to thesupport element, and a guiding mechanism associated with the referencedevice for guiding the arrangement of the latter on the cylindrical parttowards the checking condition of the apparatus.

2. Description of the Related Art

Apparatuses with these characteristics, for the diameter checking ofcrankpins rotating with orbital motion about a geometrical axis, aredisclosed in U.S. Pat. No. 6,067,721, assigned to the same assignee ofthis patent application. In particular, according to the embodimentsshown and described in the above mentioned U.S. patent, the apparatuseshave Vee-shaped reference devices that rest on the crankpin to bechecked, and guide means associated with said Vee-shaped referencedevices, for example with elements with suitably shaped surfaces forentering into engagement with the crankpin.

The embodiments shown in U.S. Pat. No. 6,067,721 guarantee excellentmetrological results and small forces of inertia, and the standards ofperformance of the apparatuses with these characteristics, manufacturedby the assignee of the present patent application, confirm theremarkable quality and the reliability of the applications.

However, in some cases there can be problems owing to the limited spaceavailable on the machine tool for mounting the checking apparatusincluding the guide elements that have surfaces with appropriateextension for guaranteeing the correct cooperation with the crankpin tobe checked.

SUMMARY OF THE INVENTION

Object of the present invention is to provide an apparatus for themetrological checking of crankpins rotating in the course of themachining in a machine tool, for example during the in-process checkingin a grinding machine for grinding crankpins rotating with an orbitalmotion, that can guarantee the same standards of performance, in termsof accuracy and reliability, as those of the apparatuses according tothe above mentioned U.S. patent and offers small layout dimensions withrespect to those apparatuses.

This problem is solved by a checking apparatus of the hereinbeforementioned type, wherein the guiding mechanism includes a limiting devicewith at least a pair of abutting surfaces adapted to engage with eachother and limit the closing movements of the reference device during theautomatic displacements towards the checking condition, and to disengagefrom each other further to the arrangement of the reference device onthe cylindrical part to be checked.

An advantage that the present invention provides is that, by virtue ofthe presence of the limiting device, the reference device is preventedfrom colliding against the grinding wheel in the event the apparatus isaccidentally controlled to displace towards the checking condition andthere is no crankpin on the worktable.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described in more detail with reference to theenclosed drawings, showing a preferred embodiment by way of illustrationand not of limitation. In said drawings:

FIG. 1 is a lateral view of a measuring apparatus mounted on thegrinding-wheel slide of a grinding machine for crankshafts, shown in arest position;

FIGS. 2 and 3 are partial, lateral views, shown in an enlarged scale andwith some details cross-sectioned, of the apparatus of FIG. 1, under adifferent operating condition;

FIG. 4 is a partial, lateral view of some components of the apparatusshown in FIG. 3, taken along the line IV-IV of FIG. 5;

FIG. 5 is a partial, front view of the apparatus of FIG. 3 mounted onthe grinding-wheel slide of the grinding machine;

FIG. 6 shows a detail of the measuring device of an apparatus accordingto the present invention for the comparative measurement of the diameterof a crankpin, that avoids interferences with the lublication hole inthe crankpin; and

FIG. 7 is a partially cross-sectional view of the measuring system ofthe apparatus.

DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to FIGS. 1 to 5, the grinding-wheel slide 1 of a computernumerical control (“CNC”) grinding machine for grinding crankshaftssupports a spindle 2 that defines the axis of rotation 3 of grindingwheel 4. The grinding-wheel slide 1 carries a support device including asupport element 5 that, by means of a rotation pin 6, with preloadedbearings—not shown—, supports a first rotating, coupling element 9. Pin6 defines a first axis of rotation 7 parallel to the axis of rotation 3of grinding wheel 4 and to the axis of rotation 8 of the crankshaft. Inturn, coupling element 9, by means of a rotation pin 10, with preloadedbearings—not shown—, defining a second axis of rotation 11 parallel tothe axes of rotation 3 and 8, supports a second rotating, couplingelement 12. At the free end of coupling element 12 there is coupled aguide casing 15 wherein there can axially translate a transmission rod16 carrying a feeler 17 for contacting the surface of crankpin 18 to bechecked. The displacements of rod 16 are detected by a measuring device,as hereinafter disclosed. At the lower end of the guide casing 15 thereis fixed a support block 19 supporting a reference device 20,Vee-shaped, adapted for engaging the surface of crankpin 18 to bechecked, by virtue of the rotations allowed by pins 6 and 10. Thetransmission rod 16 is movable along the bisecting line of theVee-shaped reference device 20.

A balance spring 21, more specifically a helical torsional spring, shownin FIG. 5, is arranged between support element 5 and the first rotatingelement 9. The spring 21, that has substantially cylindrical extension,is arranged about pin 6 and has its ends coupled to pins 13 and 14integral to support element 5 and to the first rotating, couplingelement 9, respectively. The spring 21 applies a force that tends tocause the rotation of the first coupling element 9 with respect to thesupport element 5 in a clockwise direction (with reference to FIGS.1-3), and to counterbalance the weight of the apparatus in a checkingcondition, as hereinafter described.

A guiding mechanism that, according to the following more detaileddescription, has the function of guiding reference device 20 to engagecrankpin 18, includes a limiting device 70 located between thegrinding-wheel slide 1 and the second coupling element 12, for limitingand guiding the movements of the first coupling element 9 and the secondcoupling element 12 during their rotations about the axes of rotation 7and 11 defined by pins 6 and 10.

The limiting device 70 includes bearing and guiding elements with atubular element 71 fixed to the first coupling element 9 by means ofscrews 72 and arranged parallel to said element 9, and a rigid elongateelement or stem 73, housed at the interior of tubular element 71 andguided to perform axial translations by two bushings 74 and 75 arrangedat the interior of the tubular element 71 and illustrated in FIG. 3.Internal abutting surfaces 68 and 69 limit the entity of the axialtranslations between stem 73 and tubular element 71 and a compressionspring 67 keeps the surfaces 68 and 69 one against the other when thereare no external stresses. The ends of stem 73 have a first mechanicalabutting surface 76 and a second mechanical abutting surface 77(illustrated in FIGS. 2 and 3), that are substantially plane andperpendicular with respect to the axis of stem 73. Furthermore, limitingdevice 70 includes a first reference abutment element with a pin 78coupled in an adjustable way to the support element 5 (and consequentlyto grinding-wheel slide 1), and a second reference abutment element witha pin 79 coupled in an adjustable way to the second coupling element 12(and consequently to reference device 20). The pins 78 and 79 defineconvex, in particular spherical, abutting surfaces. A differentembodiment of the invention can include abutting surfaces 76, 77, 78 and79 having different goemetrical shapes.

A return closing spring 26 is coupled between blocks 22 and 24, that arefixed, respectively, to the first (9) and to the second (12) rotating,coupling elements in the vicinity of the rotation pin 10. An adjustablepositive stop element 33, also coupled to block 22, is urged by spring26 against an abutting surface of block 24 for achieving a device forlimiting the angular position between the first (9) and the second (12)coupling elements and the associated reciprocal rotation displacements.

An additional fixed abutment element 35 is rigidly coupled to supportelement 5 and intended for cooperating with the external surface oftubular element 71 for limiting the rotations in a counterclockwisedirection (with reference to FIGS. 1-3) of the first coupling element 9about axis 7.

The crankshaft to be checked is positioned on the worktable 23, betweena spindle and a tailstock, not shown, that define the axis of rotation8, coincident with the main geometrical axis of the crankshaft. As aconsequence, crankpin 18 performs an orbital motion about axis 8.Reference number 18′ indicates the upper position that the crankpinreaches, whereas reference number 18″ indicates the crankpin lowerposition. Even though crankpin 18 rotates eccentrically about axis 8, bydescribing a circular trajectory, the trajectory of the pin with respectto the grinding-wheel slide 1 can be represented, substantially, by thearc shown with a dashed line and indicated by reference number 25. Thus,when reference device 20 rests on crankpin 18, it describes a similartrajectory, with a reciprocating motion from up to down and vice versaand at a frequency—of some tens of revolutions per minute—equal to thatof the orbital motion of crankpin 18. This is due to the fact that thechecking apparatus is carried by the grinding-wheel slide 1 that, inmodern numerical control grinding machines, machines the crankpins whilethey rotate in an orbital motion, by “tracking” the pins so as to keepthe grinding wheel in contact with the surface to be ground. Obviously,there is added, to the transversal “tracking” motion, a feed motion forthe stock removal. Thus, it is understood that the displacements of theelements forming the checking apparatus involve relatively small forcesof inertia, to the advantage of the metrological performance, limitedwear and reliability of the apparatus.

A control device, with some details shown in FIGS. 4 and 5, includes adouble-acting cylinder 28, for example of the hydraulic type. Cylinder28 is supported by grinding-wheel slide 1 and comprises a movableelement, in particular a rod 29, coupled at one end to the piston ofcylinder 28, and at the other, by means of a rotation pin 27, at anintermediate portion of a movable element, more specifically a lever 30,the latter being in turn connected to an end, and by means of rotationpin 6, to support element 5. When cylinder 28 is activated fordisplacing the piston and retracting rod 29 to the left (with referenceto FIG. 4), lever 30 rotates in a counterclockwise direction (withreference to FIG. 4) about pin 6, and the free end of lever 30 contactsa hooking element or pin 32 fixed to the first coupling element 9,causing the latter to rotate in a counterclockwise direction (FIG. 4)and the checking apparatus to displace to the rest position shown inFIG. 1. During this displacement, the abutting surface of block 24enters into contact, urged by spring 26, with the positive stop element33, thus defining a minimum value of the angle formed between the twocoupling elements 9 and 12. The retraction of the checking apparatus tothe rest position is normally controlled by the grinding machinenumerical control when, on the ground of the measuring signal of thechecking apparatus, it is detected that crankpin 18 has reached therequired (diametral) dimension. Thereafter, the machining of other partsof the crankshaft takes place, or—in the event the machining of thecrankshaft has been completed—the piece is unloaded, manually orautomatically, and a new piece is loaded on worktable 23.

When a new crankpin has to be machined, it is brought in front ofgrinding wheel 4, usually by displacing worktable 23 (in the case of agrinding machine with a single grinding wheel), and the apparatus movesto the checking condition. This occurs by controlling, by means of thegrinding machine numerical control, cylinder 28 so that rod 29 isdisplaced to the right (with reference to FIG. 4). Thus, the free end oflever 30 (that rotates in a clockwise direction—in FIG. 4—about rotationpin 6) disengages from hooking pin 32 and support block 19, throughrotations of the coupling elements 9, 12, due to the specific weight ofthe components of the checking apparatus, approaches crankpin 18, thatin the meanwhile moves according to its orbital trajectory. Morespecifically, in a first phase of the displacement the coupling elements9 and 12 integrally rotate about rotation axis 6 because spring 26 keepsthe positive stop element 33 in abutment against the surface of block24. In this phase the position of stem 73 in tubular element 71 isdefined by the resting of the internal abutting surfaces 68 and 69 urgedagainst each other by the thrust of compression spring 67. After arotation of a specific entity, and while support block 19 and referencedevice 20 perform closing movements with respect to the support element5 approaching both orbitally moving pin 18 and grinding wheel 4, thefirst mechanical abutting surface 76 of stem 73 touches pin 78 and,after a further short travel, a surface of the Vee-shaped referencedevice 20 engages crankpin 18 or does not. In the first case, further tothe full resting of device 20 on crankpin 18, the positive stop element33 detaches from the surface of block 24 and the correct cooperationbetween crankpin 18 and reference device 20 is maintained in the courseof the checking phase by virtue of the displacements of couplingelements 9 and 12 caused by the force of gravity and by the thrust ofcrankpin 18, in opposition to the force of gravity applied to theelements of the checking apparatus. The dimensions and structure of theapparatus may be designed in such a way that, for example, each of thesides of the Vee of the reference device 20 applies to crankpin 18 aforce, due to gravity, of about one kilogram.

If, on the contrary, in the course of the above-mentioned closingmovements and after contact has occurred between the first mechanicalabutting surface 76 of stem 73 and pin 78, the surface of the Vee-shapedreference device 20 does not engage crankpin 18 (and, in some cases, thedimensions of the apparatus can be such that, in the first phase of thedisplacement, the Vee-shaped reference device 20 cannot engage crankpin18), after a further short travel the second end surface 77 of stem 73touches pin 79. At this point (FIG. 2), in a second phase of thedisplacement, the positive stop element 33 disengages from the surfaceof block 24 and, while the first coupling element 9 continues to rotateabout axis 7, the second coupling element 12 rotates with respect to thefirst 9 about the second axis of rotation 11, and support block 19displaces along a trajectory that is substantially parallel to theprofile of grinding wheel 4. This displacement is due to the fact that,in this second phase of the displacement, the contact points between theplane end surfaces 76 and 77 and abutment pins 78 and 79 (the positionof these points varies as a consequence of the sliding of the abutmentpins 78 and 79 on surfaces 76 and 77) define instantaneous axes ofrotation parallel to axes 7 and 11. In other words, the first (9) andthe second (12) coupling elements, support element 5 and stem 73 definea substantially parallelogram-shaped structure in which the distanceseparating two axes of rotation with respect to the other two slightlyvaries in the course of the displacement. During this displacement, theVee-shaped reference device 20 is guided towards crankpin 18 and cancome into contact with it in any whatsoever point of its orbitaltrajectory.

The apparatus dimensions are such that contact between crankpin 18 andthe Vee-shaped device 20 cause the immediate disengagement of one of thetwo pairs of mechanical abutting surfaces defined by ends 76 and 77 ofstem 73 and abutment pins 78 and 79. FIG. 2, that refers to a conditionin which both plane end surfaces 76 and 77 of stem 73 have contacted therespective abutment pins 78 and 79, shows the position of the Vee-shapeddevice 20 with respect to the upper position 18′ that crankpin 18reaches in the course of its displacement. It is obvious that crankpin18 first meets the side of the Vee farthest from grinding wheel 4, anddevice 20 adapts its position on crankpin 18 thanks to a limitedrotation of support block 19 (in a counterclockwise direction withreference to FIG. 2), and this rotation causes the disengagement of theplane end surface 77 of stem 73 and pin 79. Hence, it is possible tokeep the correct cooperation between crankpin 18 and reference device 20in the course of the checking phase, as previously mentioned, by virtueof the displacements of coupling elements 9 and 12, caused by the forceof gravity and by the thrust of crankpin 18, in opposition to the forceof gravity acting on the elements of the checking apparatus. Thus,limiting device 70 does not interfere, during the checking phase, withthe free displacing of the coupling elements 9 and 12.

Thus, the presence of limiting device 70 enables to achieve a guideddisplacement of the Vee-shaped reference device 20 towards crankpin 18to be checked, and this displacement guarantees the dynamic engagementof the former on the latter in a correct checking condition. Limitingdevice 70, that starts operating only at a certain point of theautomatic displacement controlled by cylinder 28, and only if necessary(in other terms if the correct engagement of the Vee-shaped device 20 oncrankpin 18 in the meantime has not occurred), limits the closingmovements of reference device 20 during which the latter approachestowards grinding wheel 4, guiding its displacements along a trajectorythat is substantially parallel to the profile of grinding wheel 4. Thedisplacements guided along such trajectory take place until there isachieved the correct cooperation between Vee-shaped device 20 andcrankpin 18.

Thus, even in the event that owing to an error there is no crankshaft 34positioned in the checking position on worktable 23, and referencedevice 20 does not find any crankpin 18 on which rest, limiting device70 prevents device 20 from colliding against grinding wheel 4, andcausing imaginable negative consequences. On the other hand, aspreviously mentioned, by virtue of the disengagement of the surfaces ofthe pair of mechanical abutting surfaces 77 and 79 at the appropriatemoment in time, limiting device 70 does not limit in any way the freedisplacement of the coupling elements 9 and 12 in the course of thechecking phase, thereby guaranteeing, during that phase, the correctcooperation between the Vee-shaped device 20 (and the components of themeasuring device, coupled thereto) and orbiting crankpin 18 to bechecked.

In some cases, the forward displacement of rod 29 may be controlled sothat the approaching movement of support block 19 be temporarily stoppedin correspondence to a position close to the trajectory 25, but slightlyapart from the upper position 18′ of crankpin 18. The full extension ofrod 29 is then controlled by the grinding machine numerical control whencrankpin 18 is going to reach its upper position 18′, so that thecrankpin 18 dynamically engages reference device 20 substantially atsuch upper position 18′. This way of proceeding allows to have a lowmutual speed between the parts that come into engagement with each other(reference device 20 and crankpin 18), so providing a very soft impactbetween them.

Obviously, even the phase of displacement of the apparatus towards therest position, controlled by activating cylinder 28 for causing theretraction of rod 29, as previously described, can include two phases,as the displacement towards to the checking condition.

More particularly, depending on the position in which there takes placethe disengagement between the Vee-shaped device 20 and the surface ofpin 18, it may occur that, further to this disengagement and thesubsequent rotation of the second coupling element 12 in acounterclockwise direction (with reference to the view in FIGS. 1-3),pin 79 contacts the end surface 77 of stem 73 (the other end 76 of thelatter being already in abutment against abutment pin 78—FIG. 3), andthere takes place a phase of guided displacement towards the restposition, until, by continuing the rotation about axis 7 (in a clockwisedirection with reference to FIGS. 1-3), the distance between the convexsurfaces of pins 78 and 79 is greater than the length of stem 73 andfirst surface 77 and pin 79 disengage, thereafter surface 76 and pin 78.As a consequence of the disengagement of the surfaces of the first pairof mechanical abutting surfaces (77,79), spring 26 causes the closurebetween rotation elements 9 and 12, that is limited by contact betweenpositive stop element 33 and the surface of block 24.

In the checking condition the cooperation between crankpin 18 andreference device 20 is maintained, as above described, owing to thedisplacements of the components caused by the force of gravity. Theaction of spring 21, the stretching of which increases with the loweringof support block 19, partially and dynamically counterbalances theforces due to the inertia of the moving parts of the checking apparatusfollowing the displacements of crankpin 18. In such a way, it ispossible, for example, to avoid strong overstresses between thereference device 20 and the crankpin 18, in correspondence to the lowerposition 18″, that might tend to cause deformations of the Vee shape ofthe reference device 20. On the other side, since during the raisingmovement of the apparatus (due to rotation of the crankpin towards theupper position 18′), the pulling action of spring 21 decreases, theinertial forces tending, in correspondence to the upper position 18′, torelease the engagement between the Vee-shaped reference device 20 andthe crankpin 18, can be properly counterbalanced. It should be realizedthat spring 21 does not cause any pressure between reference device 20and crankpin 18, that mutually cooperate, as above mentioned, just owingto the force of gravity.

The coupling elements 9 and 12 are basically linear arms with geometricaxes lying in transversal planes with respect to the axis of rotation 8of the crankshaft and to the axis of rotation 3 of grinding wheel 4.However, as shown in FIG. 5, wherein there is also partially shown acrankshaft 34, in order to avoid any interferences with elements anddevices of the grinding machine, coupling element 12 comprises portions36 and 37 extending in a longitudinal direction and portions off-set indifferent transversal planes.

FIGS. 6 and 7 show some details of the measuring device of theapparatus. In FIG. 6 there is shown a crankpin 18 featuring in thecentral part, as usual, a lubrication hole 38. For the purposes ofensuring the most suitable reciprocal position between the Vee-shapedreference device 20 and the surface of crankpin 18, the guide tubularcasing 15 is symmetrically arranged with respect to the intermediatecross-section of crankpin 18. For the purposes of avoiding anyinterferences with lubrication hole 38, feeler 17 is offset with respectto the intermediate cross-section of pin 18, by means of a transversalportion 40 of transmission rod 16.

The axial displacements of transmission rod 16 with respect to areference position are detected by means of a measurement transducer,fixed to tubular casing 15, for example a transducer 41 of the LVDT orHBT type (known and herein not illustrated in the figures), with fixedwindings and a ferromagnetic core 43 coupled to a movable element, orrod 42, movable with the transmission rod 16. As shown in FIG. 7, too,the axial displacement of the transmission rod 16 is guided by twobushings 44 and 45, arranged between casing 15 and rod 16. A metalbellows 46, that is stiff with respect to torsional forces, and has itsends fixed to rod 16 and to casing 15, respectively, accomplishes thedual function of preventing rod 16 from rotating with respect to casing15 (thus preventing feeler 17 from undertaking improper positions) andsealing the lower end of casing 15.

The support block 19 is secured to guide casing 15 by means of pairs ofscrews 50 passing through slots 51 and supports reference device 20,consisting of two elements 52, 53 with sloping surfaces, whereto thereare secured two bars 54, 55. The rest position of feeler 17 can beadjusted by means of screws 50 and slots 51.

It is possible to equip one of the above described checking apparatuseswith further feelers, associated transmission rods and measurementtransducers for detecting further diameters and other dimensions and/orgeometrical or shape characteristics of the crankpin being machined.

The Vee-shaped reference device 20 can be replaced with referencedevices of a different type.

It is obvious that in a multiwheel grinding machine simultaneouslymachining a plurality of crankpins there can be foreseen just as manychecking apparatuses. Furthermore there can be foreseen othermanufacturing variants in checking apparatuses according to theinvention, e.g. as far as the structure of the limiting device isconcerned, that can include, for example, an elongate element hinged tosupport element 5 (or to the second coupling element 12), constrained toperform limited rotations with respect to the first coupling element 9,and provided with a free end for achieving a mechanical abutting surfacewith a suitable element integral with the second coupling element 12 (orto support element 5) for causing the changing from the first to thesecond phase in the displacement of the apparatus, as previouslydescribed, and guide the displacement of the reference device in saidsecond phase.

Other possible embodiments of guide means in a checking apparatusaccording to the invention include limiting devices with at least twomechanical reference abutment elements that, in the course of theapparatus displacement from the rest position to the checking condition,cooperate together for limiting and guiding the displacement of thereference device (20), but do not interfere with the displacements ofthe coupling elements (9 and 12) in the course of the crankpin checkingphase.

Other possible variants can involve the structure and the arrangement ofthe control device and/or of the balance spring 21. As an example,spring 21 can be replaced with a compression or traction spring properlyarranged between the mutually rotating parts of the apparatus, or can beremoved. In the latter case, the features of compression spring 67 (inthe limiting device 70) can be properly selected in order to perform thecouterbalancing action. In fact, in the checking condition, whileabutting surface 77 and pin 79 are disengaged, surface 76 is kept inabutment against pin 78 by spring 67. As a consequence, during thechecking operation, spring 67 applies a thrust between the supportelement 5 (through pin 78 and stem 73) and the first coupling element 9(through the tubular element 71). The thrust applied by compressionspring 67 opposes to the action of the forces of gravity, and variesduring the movements of the reference device 20, dynamicallycounterbalancing part of the forces due to the inertia of the movingparts of the apparatus, as explained above with reference to spring 21.

Furthermore, there can be foreseen an additional guide element coupledto reference device 20, achieved, for example, as disclosed in U.S. Pat.No. 6,067,721.

An apparatus according to the invention, even though particularlysuitable for the checking of crankpins in orbital motion, can obviouslybe generally utilized for checking a cylindrical part rotating about itsaxis, or about an axis parallel to its axis.

1. A method for checking the diameter of a cylindrical part, rotatingabout a geometrical axis, in a numerical control grinding machineincluding a worktable, defining said geometrical axis, and agrinding-wheel slide carrying a grinding-wheel, by means of an apparatuswith a reference device, a measuring device movable with the referencedevice, coupling elements movably connected to the grinding-wheel slideand carrying the reference device and the measuring device, and at leastone pair of mechanical abutting surfaces, the method comprising:controlling movements of the coupling elements for displacing thereference device towards the cylindrical part to be checked, themechanical abutting surfaces of said at least one pair cooperating witheach other and guiding said movements, and causing, in the course ofsaid movements, a contact between said reference device and the rotatingcylindrical part, and a disengagement of the mechanical abuttingsurfaces of said at least one pair.
 2. The method according to claim 1,wherein the reference device maintains contact with the cylindrical partto be checked substantially due to forces of gravity.
 3. The methodaccording to claim 2, wherein said forces of gravity are opposed by athrust, the latter being applied to said reference device by means of aspring.
 4. The method according to claim 1, wherein said referencedevice of the apparatus is Vee-shaped.
 5. The method according to claim1, wherein a substantially parallelogram-shaped structure is defined andguides said movements of the coupling elements.
 6. The method accordingto claim 5, wherein said substantially parallelogram-shaped structureincludes said at least one pair of mechanical abutting surfaces.
 7. Themethod according to claim 1, for checking a crankpin that is orbitallyrotating about said geometrical axis.